Fix: indented keywords would fail template expansion.

Issue description
==============
In template files, whitespaces before keywords would cause exceptions.


I don't have a JIRA issue for this, but here's a link to a discussion about this issue on the Drools user mailing list:

http://drools.46999.n3.nabble.com/Why-are-indented-keywords-in-template-causing-NPE-or-DecisionTableParseException-tp4021193.html (TL;DR - "Response: Patch welcome")


Example of indented "template" keyword
--------------------------------------------------------

```    template header
age
type
log```

Gives the exception:
org.drools.template.parser.DecisionTableParseException: Missing header 
at org.drools.template.parser.DefaultTemplateContainer.parseTemplate(DefaultTemplateContainer.java:87)


Example of indented "rule" keyword
-------------------------------------------------
```     rule "Cheese fans_@{row.rowNumber}"
when
   Person(age == @{age})```

Gives the exception:
java.lang.NullPointerException
at org.drools.template.parser.DefaultTemplateColumn.createCellCondition(DefaultTemplateColumn.java:67)

Patch description
==============

* Altered the DefaultTemplateContainer.java to trim lines before comparison.
* Added simple unit-tests to expose bug / confirm patch