String.from_char_data! is no longer available, changing to use to_string

I'm running phoenix under v0.14.1-dev and when i run the unit tests i am receiving the following when it hits the ex_conf tests:

** (UndefinedFunctionError) undefined function: String.from_char_data!/1

Doing a little digging and it looks like the from_char_data/1 has be removed under v0.14.1-dev.  I changed it to use to_string and the unit tests are now passing.

After the change, i was able to generate new phoenix projects and then run tests in/on the project.

I did not up the version dependency in mix.exs.