Convert to internal value before validation

[johtso]

Fixes #2710

How does this look?

[tomchristie]

I would consider re-jigging how this works - as it is this kinda confuses me.

Could we just alter the if data == '': check so that it also checks for whitespace only. Eg: if data == '' or self.trim_whitespace and six.text_type(data).strip() == '':

[xordoquy]

Why shouldn't we be relying upon validate_empty_values ?
Seems sensible to me.

[xordoquy]

Forget about the previous comment. Actually, it's the call to to_internal_value that does the trimming.

[xordoquy]

Can't we go with this instead:

value = super(CharField, self).run_validation(value)
if value == '':
    if not self.allow_blank:
        self.fail('blank')
return value

[tomchristie]

Superseeded by #3073.