fix parse of nested multiple binary ops (#782)

[barelsh]

Overview

Bug fix - binary operations parsed incorrectly when used multiple times in a nested constraint

Related Issue / Discussion

Issue #782

Additional Information

The inner queries list was not clened between iterations, so the expressions were appended on the previous iterations' expressions, resulting in a wrong SQL query built. (first properties were duplicated into the second condition)

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute bugfix Bug: CSW generates wrong DB queries while nesting logical operators (AND, OR) #782 to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines

[kalxas]

Thank you @barelsh for your contribution.
We will review in the following days