[csharp] Fix interfacePrefix sensitivity + default

[jimschubert]

After merging the fix with another C# change, noticed case sensitivity
issue for the true/false checks. Also noticed my original implementation
wasn't setting the default I- prefix as intended. This commit resolves
those three issues.

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

#4557 added support for customizing the interface prefix used in NancyFX Server and C# Client generators. That implementation did a case sensitive comparison in the true and false checks. It also didn't set the default I prefix unless the interfacePrefix option was supplied.

In addition to the checks from the original PR:

# Default behavior, prefix with I-
./bin/csharp-petstore.sh --additional-properties interfacePrefix=true

# Customize without the default prefix
./bin/csharp-petstore.sh --additional-properties interfacePrefix=false

# Customize using your own prefix
./bin/csharp-petstore.sh --additional-properties interfacePrefix=ZZ

Also evaluate:

./bin/csharp-petstore.sh

To ensure I- prefix default has not changed.