v0.7.5: Fixes regression in JSON struct field name

Fixes webrpc/webrpc#135

Example:

1. Given this RIDL

 message User
  - ID: uint64
       + go.tag.db = id

2. the generated code will result in

 type Nickname struct {
-   ID        uint64     `json:"iD" db:"id"`
+   ID        uint64     `json:"ID" db:"id"`
 }