Allow field names to be same as property name

This pull request fixes a bug that was preventing you from defining a property with a "field" option that has the same name as its property name.  Before this fix, the value was getting deleted and therefore not included in the query if the two names were the same.

Example:
To reproduce, define a property with a field option the same as the name of the property, then try to save it:

    myname: { type: dataTypes.TEXT, field: 'myname' }