Compare feed names case-insensitively

Feeds are matched with a case-sensitive search when adding new feed
data to a Group.  This can cause different values to be published
for the same feed in a single call to save() in a way that's not
easy to see from the code.

Avoid the problem by matching feeds case-insensitively when searching
for existing feeds in the Group data list.

Note: I added the new `strcmp_nocase` function inline in the
AdafruitIO_Group.cpp module since I didn't see any clear place to put
these kinds of utility functions in the library.  I'm open to moving
it, renaming, restructuring it or whatever.  Perhaps a better option
is to use the `String::equalsIgnoreCase()`, but constructing String
temporaries for each comparison seemed excessive to me.

Fixes adafruit#123

Author: phord

src/AdafruitIO_Group.cpp

@@ -230,6 +230,14 @@ bool AdafruitIO_Group::save() {
 /**************************************************************************/
 bool AdafruitIO_Group::get() { return _get_pub->publish("\0"); }
 
+/**************************************************************************/
+/* Helper function to compare strings case-insensitively */
+int strcmp_nocase(const char *str1, const char *str2) {
+  while (*str1 && *str2 && tolower(*str1++) == tolower(*str2++)) ;
+  auto diff = tolower(*str1) - tolower(*str2);
+  return diff < 0 ? -1 : !!diff;
+}
+
 /**************************************************************************/
 /*!
     @brief    Obtains data from feed within group.
@@ -251,7 +259,7 @@ AdafruitIO_Data *AdafruitIO_Group::getFeed(const char *feed) {
 
   while (cur_data != NULL) {
 
-    if (strcmp(cur_data->feedName(), feed) == 0) {
+    if (strcmp_nocase(cur_data->feedName(), feed) == 0) {
       return cur_data;
     }
 
@@ -314,7 +322,7 @@ void AdafruitIO_Group::onMessage(const char *feed,
 
   while (cur_cb != NULL) {
 
-    if (strcmp(cur_cb->feed, feed) == 0) {
+    if (strcmp_nocase(cur_cb->feed, feed) == 0) {
       return;
     }