Correctly support form POST with text/plain content type

[mathieucarbou]

Fixes #279

[willmmiles]

I concur with the approach, but I'd suggest replacing the macro with an inline function instead of changing the invocation. This is a good example of the fact that macros don't behave like functions. Virtually every C++ style guide suggests preferring inlines for cases like this.

static inline bool __is_param_char(char c) { return ((c) && ((c) != '{') && ((c) != '[') && ((c) != '&') && ((c) != '=')); }

[mathieucarbou]

@willmmiles : will do!