There was a change (ruby/ruby@6c70fed) in Ruby 2.6 that moved
RUBY_DESCRIPTION global constant definition out of Init_version().
Unit initialized Ruby incorrectly, so the constant was not defined.

This closes #330 issue on GitHub.

This closes #223 issue on GitHub.

According to CGI/1.1 RFC 3875:

   The server MUST set this variable; if the Script-URI does not include a
   query component, the QUERY_STRING MUST be defined as an empty string ("").

Python's PEP 333(3) allows omitting it in WSGI interface; PHP docs force no
requirements; PSGI and Rack specifications require it even if empty.

When nginx proxies requests over FastCGI, it always provides QUERY_STRING.
and some PHP apps have been observed to fail if it is missing (see issue
#201 on GitHub).

A drawback of this change (besides a small overhead) is that there will be
no easy way to tell a missing query string from an empty one (i.e. requests
with or without the "?" character); yet, it's negligible compared to the
possible benefits of wider application compatibility.

This closes #226 issue on GitHub.

Previously, the nxt_router_prepare_msg() function expected server host among
other headers unmodified.  It's not true anymore since normalization of the
Host header has been introduced in 77aad2c142a0.

The nxt_unit_split_host() function was removed.  It didn't work correctly with
IPv6 literals.  Anyway, after 77aad2c142a0 the port splitting is done in router
while Host header processing.

Library now used in all language modules.
Old 'nxt_app_*' code removed.

See src/test/nxt_unit_app_test.c for usage sample.

This closes #93 PR on GitHub.