Prevent discrepancy between the original sort and the cursor sort

[FabienChaynes]

As seen in #28 (comment) (2c.), we want to prevent discrepancies between the cursor sort options and the original sort options.

This PR raises a new Mongoid::Scroll::Errors::DifferentSortFieldsError in case of mismatch:

[1] pry(main)> cursor = Mongoid::Scroll::Cursor.from_record(Feed::Item.first, { field: Feed::Item.fields['_id'] })
=> #<Mongoid::Scroll::Cursor:0x000055dfe8ffe588
 @direction=1,
 @field_name="_id",
 @field_type="BSON::ObjectId",
 @tiebreak_id=BSON::ObjectId('541014e17aa58d8ccf000025'),
 @value=BSON::ObjectId('541014e17aa58d8ccf000025')>

[2] pry(main)> Feed::Item.desc(:created_at).scroll(cursor)
Mongoid::Scroll::Errors::DifferentSortFieldsError: 

It was inspired from raise_multiple_sort_fields_error if multiple_sort_fields?.

[FabienChaynes]

I'll add the same mechanism to the other *::Scrollable classes once we'll agree on an implementation for Mongoid::Criteria::Scrollable.

[dblock]

This LGTM.

A possibly better word than "different" is "mismatched", I think it's more idiomatic. Up to you.

Please also update README describing what happens in this case, CHANGELOG and UPGRADING.md.

Pick whether you think this is a breaking change or a fix, and either increment the minor version or the major version as part of this PR.

[FabienChaynes]

Ready for a new round of review, feel free to update any wording!

[dblock]

Almost there.

[dblock]

LGTM, needs a rebase

[dblock]

👍 merged