Harvest-API: `per_page=0` on list endpoints returns results instead of zero or an error

**Description**
Passing per_page=0 to any list endpoint returns a full page of results rather than zero results or a validation error.

**Steps to Reproduce**
GET https://harvest.data.gov/harvest_sources/?per_page=0

**Expected Behavior**
Zero results or HTTP 422.

**Actual Behavior**
Returns a full page of results as if the parameter was ignored.

**Notes**
Confidence: MEDIUM -- tested on /harvest_sources/ only. Needs confirmation on other list endpoints. Also test per_page=-1 for consistency.