Fix for issue #71 (Content-Length injection into header fields)

[geldrin]

Fixed the issues I reported previously in ticket #71 .

The prepHeaders() method now

• checks if 'Content-Length' parameter needs to be added (when you pass the second param)
• checks if the parameter exists
• inserts 'Content-Length' parameter regarding the structure of the array (numerically indexed or associative)

Also fixed _isNumericallyIndexedArray() so it will return FALSE when the supplied array's indexes are mixed.