HttpRequestBodyToParamMap should transform GET method

• User impact:

  Medium
• Similar Issues:

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In org.mule.transport.http.transformers.HttpRequestBodyToParamMap, method public Object transform(MuleMessage message, String encoding), following code seems to be invalid:
boolean isUrlEncoded = contentType.startsWith("application/x-www-form-urlencoded");
It throws Null Exception when invoked with GET HTTP method.
Suggested change:
boolean isUrlEncoded = (contentType!=null)? contentType.startsWith("application/x-www-form-urlencoded") : false;