Possible contribution: Allowing PATCH on resource endpoints

[Jeremy Solbrig]

I am considering making a contribution to Python-eve, but am hoping to get a bit of feedback before putting in the work.  I'd hate to put work in on this only to find that it is not wanted or that I am going about this the wrong way.

My colleagues have need of the ability to perform a mass update (i.e. update_many).  I can add this functionality via a custom endpoint, but that seems clumsy when I can just add the functionality to the resource endpoints themselves.  This would involve allowing operators in update documents (currently defaults to "$set" on all fields in eve/io/mongo.py) and allowing queries in updates.  To do this, I would do the following:

1. eve/flaskapp.py
1. Add "PATCH" to `supported_resource_methods`.
2. eve/endpoints.py
1. Edit collections_endpoint to call `update` for `PATCH` requests.
3. eve/io/mongo.py
1. Modify `update()` to:
1. If no operator is present in the first layer of the document, add the "$set" operator (this is currently done in all cases).
2. If operators are present in the first layer of the document, just pass the document as is.
4. eve/io/validation.py
1. Add handling for operators to `Validate.validate_update()`.  

I have mapped out how to add handling for operators to `Validate.validate_update()`, but am unsure if my approach is the best approach.  Maybe I can make better use of cerberus for this, but I am not sure how to do so.  To do this, I would attempt to make the best use of cerberus that I can.  

There are several things that need to be done to prepare the document for validation through cerberus.  There are a few different cases that need to be handled (including some that I am avoiding for now).  To handle these, a new document would be constructed for validation as though the $set operation is being performed.

1. Scalar operators
1. $inc, $min, $max, $mul, $set, $setOnInsert: 
1. Remove the operator
2. Move the field and value up a level
3. Validate the field and value as if they are going to be used in a $set operation
4. {"$min": {"field1": 1}} becomes {"field1": 1} for validation.
2. $currentDate: Leave unhandled for now, until I have a better understanding of cerberus.Validator.
2. Array Operators
1. $addToSet, $push, $pull, $pullAll: 
1. Remove the operator
2. Move the field and value up a level
3. Turn any scalars into arrays
4. Validate the field and value as if they are going to be used in a $set operation
5. {"$push": {"field1": 1}} becomes {"field1": [1]}
6. {"$pullAll": {"field1": [1, 2, 3, ...]}} becomes {"field1": [1, 2, 3, ...]}}
2. $pop:
1. Check that the value is either 1 or -1
2. Remove from the document prior to validation
3. Operators to skip for now, but which need to be handled later:
1. Scalar Operators: $currentDate
2. Array Operators: $, $[], $[<identifier>]
3. Modifiers: $each, $position, $slice, $sort, $bit

I think this is the full set of required modifications and would be very happy to hear some feedback on this.