net: http: fix http client request "Host" header

RFC-7230 "HTTP/1.1 Message Syntax and Routing" Section 5.4
describes the "Host" header formatting.  If Zephyr user
specifies a host string as a part of the HTTP client request
structure, we end up sending an incorrect HTTP header due
to a missing "Host :" text.

Fix this by prepending "Host: " to the header data before
the user supplied host string.

Signed-off-by: Michael Scott <michael.scott@linaro.org>