debug: fix RPC params of method CreateBreakpoint

[MarcoQin]

When to clear breakpoint after debug process started, the debugger will exit with status 0. Correct the RPC method's param type can fix this.

[bhcleek]

I'm not sure what this fixes. Can you provide a description of the problem that this solves?

[MarcoQin]

I encountered a problem when I did the following steps:

1. :GoDebugStart
2. Use :GoDebugBreakpoint to add a breakpoint on a line
3. :GoDebugContinue
4. Use :GoDebugBreakpoint again to clear the breakpoint I set previously
5. The status bar shows: vim-go [debug] SUCCESS, and I noticed the dlv process exit (maybe killed by the job, because the jsonRPC did not correct responsed)

The RPCServer.ClearBreakpoint method receives an int id and a string name, but the type of l:found.id is string. So I just cast it to int, and the error gone.

[bhcleek]

Thank you for contributing @MarcoQin.

The underlying issue here would be ideally resolved by executing a function that provides the id as a number, sign_getplaced(), but that function is not available until Vim 8.1.0614 and vim-go currently supports > Vim 8.0.1453 and therefore must rely on normal mode commands, which return a string, so this is indeed the right fix.