Support non-value attributes in HTML5 by setting their value to True, not ''

[danielrozenberg]

This will change the following:

{{ form.field1|attr:'required' }}

From:

<input ... required="">

To:

<input ... required>

The former can still be generated by writing:

{{ form.field1|attr:'required:' }} 

This is the same exact change that I created in the pull request in bitbucket, which you asked me to create here in github instead. I also added test cases per your request, which I hope you will find satisfactory.

Cheers!

[danielrozenberg]

I didn't realize this was automatically rejected by your build system until now 😮
Looks like this fails on versions prior to 1.8. What do you think is the best approach to this? It's a pretty useful feature but it relies on the backend to support non-value attributes. I can modify the test cases to just pass on Django versions < 1.8

[camilonova]

@danielrozenberg If you are willing to consider this again, please make a new PR. Thanks.