Fix DateTimeZone setup: fall back to gmt_offset; php notice fixes

[adamsilverstein]

Description of the Change

When timezone_string is not available (empty), fall back to gmt_offset when constructing DateTimeZone for file name.

Also includes some PHP notice fixes and some whitespace changes my editor made (removing stray spacing).

Fixes #5.

Verification Process

To test this bug, set your WordPress timezone to a "manual" offset instead of a 'named' timezone (under settings->general):

Once set, try exporting a print publication. Export will fail and an error will show:

After this fix, a zip file will download containing the export.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues