Suggestion fix for #16

[iomz]

As commented on PR #16 I've made the following changes:

• merge the scalar value fields switch to fieldToDataSchema
• split unit test for fieldToDataSchema() for each normal field type
• move the test file so that they will be tested by cmd/prototd/main_test.go
• remove the manual JSON (un)marshaller helpers

Notice that the cmd/prototd/test/scalar-value-fields/output.jsonld file makes the current test to be failing, because there's no RPC function is in the test input (should be fixed after the other feature is implemented).

[iomz]

@S-Eibl I have fixed the linter with #19 Please ignore the failed check.