Add field optional dehydrate method param

[cocorocho]

Problem

Defining custom methods for dehydration of the field

I have had situations where I had to dynamically generate new fields during export. In this case I wasn't able to pre-define dehydration methods in ModelResource so I added dehydrate_method parameter to Field which solved this issue.

Solution

I have added dehydrate_method parameter to fields.Field so custom methods

Acceptance Criteria

Have you written tests? Have you included screenshots of your changes if applicable?

I have written tests.

Did you document your changes?
Added docstrings.

Let me know if anything needs to be changed!

[coveralls]

Coverage: 100.0%. Remained the same when pulling 4c8ecab on cocorocho:add-field-optional-dehydrate-method-param into 51d703b on django-import-export:main.

[matthewhegarty]

Thanks for this - looks great. I had a couple of minor comments.

[cocorocho]

I have made the changes.

Also deleted the deprecation warning from exceptions module.

Thanks for the suggestions and explanations regarding the changes, if anything else needs to be changed let me know!

[matthewhegarty]

Looks great, almost ready to merge. Re backticks, I suggest remove these from the FieldError message (sorry I was not clear about that). Error messages can be written to log files and although I cannot see how, I wonder if having backticks could be exploitable, therefore I suggest to use single quotes. Please could you correct it?

Feel free to add your name to AUTHORS (optional), and then I will merge.

[cocorocho]

Looks great, almost ready to merge. Re backticks, I suggest remove these from the FieldError message (sorry I was not clear about that). Error messages can be written to log files and although I cannot see how, I wonder if having backticks could be exploitable, therefore I suggest to use single quotes. Please could you correct it?

Feel free to add your name to AUTHORS (optional), and then I will merge.

Ah lol ok now it makes sense. Will make these changes

[matthewhegarty]

Great thanks