Support ":" as separator for fractional seconds in the absence of a zone offset

In #117  support was added to parse timestamps of the form:
```
"2012-08-17T18:31:59:257+0100"
``` 

However, parsing timestamps that use ":" as a separator but do not have a time zone offset fails.  Attempts to parse the following:
```
"2012-08-17T18:31:59:257"
```
Fail with:
```
parsing time "2012-08-17T18:31:59.257" as "2006-01-02T15:04:05.257": cannot parse "" as "."
```