Add Content-Type headers for JSON data

[thealanjason]

Based on UM-Bridge/UMBridge.jl#30, the MATLAB client expects the Content-Type header to be application/json.

Since we are using JSON as the the data interchange format in the UMBridge ecosystem, it would make sense to update the Content-Type header uniformly for all implementations.

For Python: need to add content_type argument (based on docs) to each of the Response function calls (for eg. here).

For Matlab: need to check implementation

For Julia: handled by UM-Bridge/UMBridge.jl#30

For C/C++: need to check implementation