Fix FindBugs findings

Bug: return value of java.math.BigDecimal.add(BigDecimal) ignored in
new net.madz.lifecycle.engine.CallbackTestMetadata$Invoice(BigDecimal)
The return value of this method should be checked. One common cause of
this warning is to invoke a method on an immutable object, thinking
that it updates the object. For example, in the following code fragment,

String dateString = getHeaderField(name);
dateString.trim();
the programmer seems to be thinking that the trim() method will update
the String referenced by dateString. But since Strings are immutable,
the trim() function returns a new String value, which is being ignored
here. The code should be corrected to:

String dateString = getHeaderField(name);
dateString = dateString.trim();
Confidence: High, Rank: Scariest (3)
Pattern: RV_RETURN_VALUE_IGNORED
Type: RV, Category: CORRECTNESS (Correctness)