Let users disable sending Content-Type with empty requests with URLConnection conduit

If no Content-Type is set for empty requests then HttpUrlConnection:

• sets a form Content-Type for empty POST
• replaces custom Accept value with a wildcard if HTTP proxy is used

To bypass it CXF sets a wildcard CT for empty requests which causes issues if the target server does not accept such a wildcard.

CXF RS code can override it with a preferred CT if needed, but sending CT with GET is better be avoided so a new property "set.content.type.for.empty.request" is introduced (note no 'wildcard' in the property name to let us default to something else if needed).

The property is set to true by default. If a user sets it directly to false then CT will be dropped. Hopefully this property can be disabled by default in the future.

Note the issue applies to a default non-async conduit only. Setting the property is not needed if it is the async conduit.