Fixes issue #345 - Whitespace preservation should apply to all nested text

[patrick-steele-idem]

Description

Currently when a node/tag is marked with "preserve-whitespace", the whitespace is being preserved for only directly nested text. The expected behavior should be that whitespace should be preserved for all directly nested text and any text nested at any level below the node.

Motivation and Context

Issue #345

Checklist:

• My code follows the code style of this project.
• I have updated/added documentation affected by my changes.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage increased (+0.02%) to 84.152% when pulling 0521933 on issue-345 into d433ac2 on master.