Class JsonFormatWriter can create invalid JSON representation of OEntity object

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The Class JsonFormatWriter creates an invalid JSON representation in case that 
an OEntity object doesn't contain any OProperty instances but 
ORelatedEntityLink instances. 
This might occurs on requests which uses projection ($select operator) only on 
navigation properties like 

http://localhost:8080/myodataservice/odata.svc/Products?$select=Supplier&$format
=json

The JsonFormatWriter will then create following invalid JSON representation of 
the OEntity object:

{
"d" : {
"results" : [
{
"__metadata" : {
"uri" : 
"http://localhost:8080/myodataservice/odata.svc/Products('c40423f4-6400-4bd1-b46
0-7f3606429c2e')", "type" : "ODataDemo.Product"
}, , "Supplier" : {
"__deferred" : {
"uri" : 
"http://localhost:8080/myodataservice/odata.svc/Products('c40423f4-6400-4bd1-b46
0-7f3606429c2e')/Supplier"
}}}]}}

The JSON representation is invalid because of the two back-to-back comma 
between the "__metadata" and the "Supplier" part. \

This might causes an MalformedJsonException in case you try to parse such an 
JSON string. 

The problem seems to be that the method writeResponseLink() in the class 
JsonFormatWriter always first writes a separator (line 262) and therefore 
expects that the OEntity object also contains instances of type OProperty. 

What steps will reproduce the problem?
1. Make a request with a projection ($select operator) on only a navigation 
property and use JSON as a return format 

What version of the product are you using? On what operating system?
Version 0.8.0-SNAPSHOT

Original issue reported on code.google.com by andyfich...@gmail.com on 21 Jan 2013 at 7:19