curlhttpsrc: add property to set HTTP method

To allow curlhttpsrc to use HTTP methods other than GET
(e.g. HEAD), add a "method" property that is a string. If this
property is not set, GET is used.

This property is used by gst_uri_downloader_set_method() when
uridownloader is requested to perform a HEAD request. This
feature of uridownloader is used by dashdemux when the http-head
UTCTiming method is used.

This commit ports this functionality from souphttpsrc to
curlhttpsrc. See https://bugzilla.gnome.org/show_bug.cgi?id=752413
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