Removing charset setting when setting default header

[mpark86]

Removed charset setting charset=utf-8 as this replaces all content-type settings unless set manually

[MrVine]

@mpark86 when request contains JSON body the default value of Content-Type header should be application/json; **charset=utf-8**, because UTF-8 is the default encoding for JSON format in accordance to RFC 7159 Section 8.1:

JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default
encoding is UTF-8, and JSON texts that are encoded in UTF-8 are
interoperable in the sense that they will be read successfully by the
maximum number of implementations; there are many implementations
that cannot successfully read texts in other encodings (such as
UTF-16 and UTF-32).

[chinesedfan]

See discussions in #2154 (comment).