IntegerField should check type when process data

[whusnoopy]

Core fields like IntegerField will check type on process_formdata by reload from base Field, but not check on process_data in current version. If we pass unsupported type in data for Form, like a string 'x', the form won't raise any error and will pass the origin string as value for user, it doesn't make sense for this case.

As BooleanField did in process_data, IntegerField should do the same thing, so I add the progress and test case for IntegerField.

Maybe other fields in fields/core.py like FloatField should do type check in progress_data also.

[ftm]

Thank you for adding this fix, apologies for not accepting the PR for over a year.