Calendar to string test fails

Test ToFromConversionTests.testToString fails on non English systems because of system locale-dependent formatter. Proposed patch uses formatter to expect the right string.

```
[testng] FAILED: testToString
[testng] java.lang.AssertionError: expected [Jun 1, 2013] but found [1-giu-2013]
[testng]     at org.testng.Assert.fail(Assert.java:94)
[testng]     at org.testng.Assert.failNotEquals(Assert.java:494)
[testng]     at org.testng.Assert.assertEquals(Assert.java:123)
[testng]     at org.testng.Assert.assertEquals(Assert.java:165)
[testng]     at com.google.refine.tests.expr.functions.strings.ToFromConversionTests.testToString(ToFromConversionTests.java:120)
```