Fix DepProxy#== undefind method error

DepProxy#== crashed with an undefind method error
for anything other than a DepProxy class or nil
as parameter. This was caused that it was assumed
only DepProxy instances or nil can get passed as
parameter. This commit implements checking the class
as well and returns false if the classes are not
the same.

Fixes jruby/jruby#5280 travis-ci/travis-ci#9994

lib/bundler/dep_proxy.rb

@@ -14,7 +14,7 @@ def hash
     end
 
     def ==(other)
-      return if other.nil?
+      return false if other.class != self.class
       dep == other.dep && __platform == other.__platform
     end
 

spec/bundler/dep_proxy_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe Bundler::DepProxy do
+  let(:dep) { Bundler::Dependency.new("rake", ">= 0") }
+  subject { described_class.new(dep, Gem::Platform::RUBY) }
+  let(:same) { subject }
+  let(:other) { subject.dup }
+  let(:different) { described_class.new(dep, Gem::Platform::JAVA) }
+
+  describe "#eql?" do
+    it { expect(subject.eql?(same)).to be true }
+    it { expect(subject.eql?(other)).to be true }
+    it { expect(subject.eql?(different)).to be false }
+    it { expect(subject.eql?(nil)).to be false }
+    it { expect(subject.eql?("foobar")).to be false }
+  end
+end