Fix schema mapping if driver includes default fields

[rbuffat]

Problem:
Some drivers create fields by default. Fiona is currently not aware of these fields. Additionally, some drivers modify the field names (e.g. Shapefile has a length restriction for field names). Fiona maintains a schema mapping between field names in the schema provided to Fiona, and the resulting field names. However, as the default fields are not considered, the resulting mapping is wrong (see #916).

This PR fixes the schema mapping if a driver contains default fields. This is kind of a minimal solution, as the existence of default fields also influence the schema validation, which is not touched with this PR. This PR does not modify that only the fields in the schema provided to Fiona are validated.

Currently, schema validation requires that records include all fields of the schema. However, drivers include also "optional" default fields, that do not need to be present. E.g. the default fields for the GPX driver are:

        'GPX': {'properties': OrderedDict([('ele', 'float'),
                                           ('time', 'datetime'),
                                           ('magvar', 'float'),
                                           ('geoidheight', 'float'),
                                           ('name', 'str'),
                                           ('cmt', 'str'),
                                           ('desc', 'str'),
                                           ('src', 'str'),
                                           ('link1_href', 'str'),
                                           ('link1_text', 'str'),
                                           ('link1_type', 'str'),
                                           ('link2_href', 'str'),
                                           ('link2_text', 'str'),
                                           ('link2_type', 'str'),
                                           ('sym', 'str'),
                                           ('type', 'str'),
                                           ('fix', 'str'),
                                           ('sat', 'int'),
                                           ('hdop', 'float'),
                                           ('vdop', 'float'),
                                           ('pdop', 'float'),
                                           ('ageofdgpsdata', 'float'),
                                           ('dgpsid', 'int')]),

E.g. thegeoidheight element is defined with minOccurs="0" in https://www.topografix.com/GPX/1/1/gpx.xsd

[coveralls]

Coverage increased (+0.04%) to 84.294% when pulling d17f8db on rbuffat:default_fields into 01cfc24 on Toblerity:maint-1.8.

[sgillies]

@rbuffat thank you! @snorfalorpagus thanks for the review!