fix: serialization of from parameter

[floriangantner]

see #243

Description

Please describe briefly your pull request.

• fixes serialization of from parameter in marshmallow. Before the attribute in the response was returned as "from_"

• usage of load_from and dump_tohas been replaced by data_key in marshmallow 3.0.0.b8

• additional usage of parameter attribute for the special case for variable from See marshmallow docs , Chapter Field.attribute and it's Note: This should only be used for very specific use cases such as outputting multiple fields for a single attribute, or using keys/attributes that are invalid variable names, unsuitable for field names. In most cases, you should use data_key instead.

• add new test case for response when from date is after until date

• enhance existing test cases to check for the from and until attribute in the response.

• improves overall test coverage by reducing missing statements from 60 to 59

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