make header-fields case-insensitive

[patrickjahns]

In order to comply with rfc7230 headerfields are now parsed case-insensitive and retrieved case-insensitive

fixes #672

[hreintke]

@patrickjahns :
I agree with the update/patch.
Formally there is backward compatibility issue. Applications which rely on the (incorrect) case sensitivity will not work anymore.
What is your opinion on this ?

[patrickjahns]

@hreintke
To be honest I didn't think of backward compatibility issues.
Generally any existing code would still work as before, no matter the spelling (i.e. getHeader("Content-Length"); getHeader("content-length"); getHeader("cOnTeNt-LeNgTh"); will all return the same.

While header names are case-insensitive, the values of the header fields are case-sensitive and thus nothing has changed from the previous behavior.

Is there any planned release notes for the upcoming Sming release?