Add missing template method implementations (yhirose#1013)

When using the split version of httplib.h the templated implementation
of e.g. Client::set_connection_timeout ends up in httplib.cc and
therefore results in a linker error since the needed template
specialization has not been instantiated. Fix this by moving the
implementation of template methods into the part that ends up in
httplib.h after the split.

Fixes yhirose#1008.

Author: jrosdahl