MINOR: httpclient/lua: supports headers via named arguments

MINOR: httpclient/lua: supports headers via named arguments

Migrate the httpclient:get() method to named arguments so we can
specify optional arguments.

This allows to pass headers as an optional argument as an array.

The () in the method call must be replaced by {}:

	local res = httpclient:get{url="http://127.0.0.1:9000/?s=99",
	            headers= {["X-foo"]  = { "salt" }, ["X-bar"] = {"pepper" }}}