Issue title: FHIRDate.as_json uses original instead of isostring
 Issue body: From fhirdate.py
```
    def as_json(self):\n
        if self.origval is not None:
            return self.origval
        return self.isostring
```
Sample code
```
date = FHIRDate('20190527T210000')
print(date.as_json())
print(date.isostring)
```
outputs
```
20190527T210000
2019-05-27T21:00:00
```
as_json() function should output the same result as isostring
 Comments: 
 Comment 0: Same as https://github.com/smart-on-fhir/fhir-parser/issues/30
{'source': 'GithubIssues', 'repo_name_with_owner': 'smart-on-fhir/client-py', 'issue_id': 449038227, 'repo_stars': 458.0}