Expression Engine: Allow case-insensitive expression function name processing

[thomasknoell]

The processing of an expression containing an expression function is issuing an error if the expression function name is not as defined in the expression function list. For example, in the expression function list, the name of the function to concatenate two strings is defined "Concat". If the user would use the term "concat" instead, the Expression Engine would issue an exception indicating an unknown function.