This was mostly the same as the other commits, but there was actually one reference to
the functions in to_nonconst_iterator.h in http_protocol_handler.inl. All the other
files modified in this commit were to remove extraneous includes of to_nonconst_iterator.h.
In http_protocol_handler.inl, to_nonsonst_iterator() (misspelled?) was used to convert a
string::const_iterator to a nonconst iterator in order to use it in a call to
string::erase(). However, this in not necessary as of C++11, so I removed the conversion
altogether.
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