Ignore inherited methods in BinData field-name guard (#261)

[arpitjain099]

Fixes #261.

Problem

Loading ruby_smb after a mocking library has patched Object raises a
SyntaxError from BinData while the gem's own struct definitions are being
built:

SyntaxError: field 'stub' shadows an existing method in RubySMB::Dcerpc::Request

The common trigger is an RSpec suite configured with config.mock_with :rspec,
because rspec-mocks adds a #stub helper to BasicObject/Object. If that
happens before require 'ruby_smb' (a normal spec_helper.rb ordering), the
inherited #stub collides with the legitimate :stub field on
RubySMB::Dcerpc::Request.

The root cause is in BinData's field-name guard
(BinData::DSLMixin::DSLFieldValidator#name_shadows_method?), which calls
@the_class.method_defined?(name). Without the second argument,
method_defined? also returns true for methods inherited from ancestors such
as Object, Kernel and Hash, so any field whose name matches an inherited
helper method is wrongly rejected.

Fix

This adds a small compatibility shim (lib/ruby_smb/compatibility.rb, required
immediately after require 'bindata') that narrows the guard to
@the_class.method_defined?(name, false). The false restricts the check to
methods defined directly on the struct class, which is what the guard is meant
to enforce.

Why this does not weaken the guard:

• Field accessors are installed per instance with define_singleton_method, so
  an inherited method that happens to share a field's name never gets in the way
  at runtime. Only a method defined directly on the struct class is a genuine
  collision, and that case still raises.
• Names that clash with Hash methods or BinData DSL internals are handled by a
  separate, independent check (name_is_reserved? against
  BinData::Struct::RESERVED). That list is unchanged, so reserved names such as
  keys still raise.

This is purely a compatibility/loosening of which field names are accepted. It
does not rename, retype, or otherwise change any BinData data definition, so the
versioning note in CONTRIBUTING.md (minor bump on BinData definition changes)
does not apply here.

Tests

spec/lib/ruby_smb/compatibility_spec.rb covers all three branches:

• an inherited method that shares a field name no longer blocks the field
  (the Field 'stub' shadows an existing method in RubySMB::Dcerpc::Request while unit testing #261 regression),
• a field that shadows a method defined directly on the struct still raises, and
• a field that collides with a BinData reserved name (keys) still raises.

Verification Steps

• bundle install

rake spec

• rake spec
• VERIFY no failures

Manual scenario

require 'rspec/mocks/standalone'   # adds Object#stub, as `mock_with :rspec` does
require 'ruby_smb'                  # previously raised "field 'stub' shadows an existing method"
puts 'loaded ruby_smb cleanly'