Added option to retrieve boolean response from JSON content

[padcom]

Hi,

sometimes the content returned is a constant true/false. Since there is no way to set the content type for TextResource I've created the "toBoolean" method on JSONResource that simply casts the unmarshaled object to boolean.

I need this since the service I am integrating with is very strict in the content it accepts and can produce (application/json and nothing else) and returns a true/false value.

Best regards, Matthias.

[padcom]

I've had problems building the solution because of some GPG plugin problem but I've tested the pull request otherwise directly from Eclipse and it worked OK.