Fix pandas.DataFrame support in core._PrintResult

[paul-ada]

• and add HasCustomRepr check in value_types

Issue:
When calling Fire on a function that returns a pd.DataFrame, the _PrintResult function in core.py prints the manual for DataFrames from the pandas Docs to stdout.

if value_types.HasCustomStr(result): doesn't recognize the DataFrame's custom __str__ as it is simply seemingly inherited from the custom __repr__ for DataFrames.
See pandas.core.frame.DataFrame.__repr__ which calls .to_string()).

Due to this, the _PrintResult function displays the DataFrame's helptext.HelpText instead.

This implementation should fix the issue.

Edit: Only seeing now that there is a related issue: #274

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[dbieber]

Thanks for the PR @paul-ada! It may be some time before we process it, but it is already appreciated.