Uncertainty interval with days between returns wrong low interval

[bryantaustin13]

To duplicate run the following:

<?xml version="1.0" encoding="utf-8"?>
<tests xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://hl7.org/fhirpath/tests" xsi:schemaLocation="http://hl7.org/fhirpath/tests ../../testSchema/testSchema.xsd" name="fails_date_interval_Test">
    <group name="AllOfThem" version="1.0">
      <test name="DateTimeDurationBetweenUncertainInterval2" version="1.4">
        <expression>months between DateTime(2005) and DateTime(2006, 5)</expression>
        <output>Interval[4, 16]</output>
      </test>
    </group>
</tests>

actual value returned is Interval[5, 16], It should be Interval[4, 16] because it should count whole months, not partial months, and Dec 31, 2005 to May 1, 2006 is 4 whole months..