Fix #632: calling functions to*() should output by default to stdout

[juarezr]

This PR has the objective of fixing #632.

Changes

1. Added a new test for checking output to stdout by default
2. Fixed the regression on all to*() functions

Checklist

Use this checklist to ensure the quality of pull requests that include new code and/or make changes to existing code.

• [X] Source Code rules apply:
  • [X] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behavior
  • [ ] All changes are documented in docs/changes.rst
• [X] Versioning and history tracking rules apply:
  • [X] Using atomic commits when possible
  • [X] Commits are reversible when possible
  • [X] There are no incomplete changes in the pull request
  • [X] There is no accidental garbage added in the source code
• [X] Testing rules apply:
  • [X] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [X] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [X] Ready to review
  • [X] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 3525847841

• 10 of 12 (83.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 91.116%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
petl/io/sources.py	4	5	80.0%
petl/test/io/test_sources.py	6	7	85.71%
<!--	Total:	10	12	83.33%	-->

Totals
Change from base Build 3131975355:	-0.001%
Covered Lines:	12758
Relevant Lines:	14002

---

💛 - Coveralls