Tokenizer respects filters when char_level is True

[paw-lu]

Summary

As outlined in #301, this PR makes keras.preprocessing.text.Tokenizer remove the characters in the filters argument if char_level=True.

Closes #301.

Behavior before

❯ tokenizer = keras.preprocessing.text.Tokenizer(char_level=True, filters="e")
❯ tokenizer.fit_on_texts("ae")
❯ tokenizer.word_index
{'a': 1, 'e': 2}  # "e" is tokenized

Behavior after

❯ tokenizer = keras.preprocessing.text.Tokenizer(char_level=True, filters="e")
❯ tokenizer.fit_on_texts("ae")
❯ tokenizer.word_index
{'a': 1}  # "e" is not tokenized

Closes #301

Related Issues

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)