[Matcher] Fix misbehaviour on validate_content_type_of matcher (#201)

[Mth0158]

Closes #201

The error was due to #183 that implemented a check on the custom_message matcher for content_type.
It was:

• always looking to check the validity of the custom_message matcher even if not present
• looking on all errors of the field (meaning not content_type errors also) to find this message

Both these behaviours have been removed.

[Mth0158]

@igorkasyanchuk I would love to work a bit more on the message matcher, I think a lot of things could be refactored, we could also improve reliability with a better test suit on this part. What do you think?

[igorkasyanchuk]

@Mth0158 Sure, do what you want and what can be good for the project. I don't have much time to focus on this project so if you can support it would be great!