Handle keywords with duplicate title

Regarding `14eadbd9d654ad9a172f566f4f617bc5756be9d5`

I had two keywords in my db with titles "test" and "Test".

I went to edit a blog post from the admin, added keyword `test` and clicked on save button. An ajax call was made to `admin_keywords_submit`.
This view was using `title_iexact` in a get call. But both `test` and `Test` come as result of `iexact`, so it threw MultipleObjectsReturned. So, changed it to say `title_exact`.

Regarding `4dbf57f9912d766b6cc2ef43d87dd1cadb95007a`

What I can guess from `admin_keywords_submit`, you don't want two keywords to be in the system with exact same title i.e only one keyword with title `Now` should be there in db. If someone tries to create it second time, it should not be allowed.