Remove template from HTTPHeader constructor
Remove template from HTTPHeader constructor

Currently, the HTTPHeader constructor is templated with the types for
the header name and the value, but the actual name and value members are
of std::string type. It is currently not possible to have a different
type for the value for example.

This commit suggests removing the template for the constructor and
making the parameters to to be of type std::string instead.  I'd be
happy to look at a templated version but was not sure if it was worth
the effort as this is "just" in an example.

ngtcp2/ngtcp2,ngtcp2/ngtcp2,ngtcp2/ngtcp2