fix: Normalize header handling for all HeaderInit types

[AlexanderSlaa]

Description:
This pull request addresses an issue where filtering custom headers using Object.entries(options.headers) fails when the headers are not provided as a plain object (for example, when using a Headers instance or an array of key-value pairs).

Changes Introduced:

• Added a new utility function normalizeHeaders that converts any valid HeaderInit (i.e. a Headers instance, an array of key-value pairs, or a plain object) into a plain object.
• Updated the custom header filtering logic to work reliably by first normalizing options.headers before filtering out the default headers.
• Included JSDoc documentation for the normalizeHeaders function.

Testing & Impact:

• Manually verified that the new implementation correctly processes different header types.
• Ensured that the filtering logic now works uniformly regardless of the header format.
• This improvement ensures better compatibility with the Fetch API and reduces potential errors when using custom headers.

[StevenSlaa]

@BruceMacD Could you take a look at this?

[BruceMacD]

Thanks for fixing this, and sorry for the delayed response.

This is looking good, but can we also add a test in test/utils.test.js to document this behavior?

Something like this:

import { describe, it, expect, vi, beforeEach } from 'vitest'
import { get } from '../src/utils'

describe('get Function Header Tests', () => {
  const mockFetch = vi.fn();
  const mockResponse = new Response(null, { status: 200 });

  beforeEach(() => {
    mockFetch.mockReset();
    mockFetch.mockResolvedValue(mockResponse);
  });

  const defaultHeaders = {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
    'User-Agent': expect.stringMatching(/ollama-js\/.*/)
  };

  it('should use default headers when no headers provided', async () => {
    await get(mockFetch, 'http://example.com');
    
    expect(mockFetch).toHaveBeenCalledWith('http://example.com', {
      headers: expect.objectContaining(defaultHeaders)
    });
  });

  it('should handle Headers instance', async () => {
    const customHeaders = new Headers({
      'Authorization': 'Bearer token',
      'X-Custom': 'value'
    });

    await get(mockFetch, 'http://example.com', { headers: customHeaders });

    expect(mockFetch).toHaveBeenCalledWith('http://example.com', {
      headers: expect.objectContaining({
        ...defaultHeaders,
        'authorization': 'Bearer token',
        'x-custom': 'value'
      })
    });
  });

  it('should handle plain object headers', async () => {
    const customHeaders = {
      'Authorization': 'Bearer token',
      'X-Custom': 'value'
    };

    await get(mockFetch, 'http://example.com', { headers: customHeaders });

    expect(mockFetch).toHaveBeenCalledWith('http://example.com', {
      headers: expect.objectContaining({
        ...defaultHeaders,
        'Authorization': 'Bearer token',
        'X-Custom': 'value'
      })
    });
  });

  it('should not allow custom headers to override default User-Agent', async () => {
    const customHeaders = {
      'User-Agent': 'custom-agent'
    };

    await get(mockFetch, 'http://example.com', { headers: customHeaders });

    expect(mockFetch).toHaveBeenCalledWith('http://example.com', {
      headers: expect.objectContaining({
        'User-Agent': expect.stringMatching(/ollama-js\/.*/)
      })
    });
  });

  it('should handle empty headers object', async () => {
    await get(mockFetch, 'http://example.com', { headers: {} });

    expect(mockFetch).toHaveBeenCalledWith('http://example.com', {
      headers: expect.objectContaining(defaultHeaders)
    });
  });
});

[AlexanderSlaa]

@BruceMacD Added the test and implemented your suggestion.

[BruceMacD]

Thanks for fixing this, the tests failed due to unrelated config issues, so I'm going to merge this one through.