filters/thread_filter: don't serialise IO objects

Fixes #204 (Airbrake gem segfaults on Circle CI)

The main reason to use `inspect` here was to avoid a bug with IO objects
in Rails apps.

I stumbled upon an edge case while running the thread filter against a
Rails app. The app would stuck because `io_obj.to_json` would never
return. The `io_obj` was open. Closed IO objects raise IOError, which
we already handle.

Given that we stop using `inspect`, I think this also fixes #204.

lib/airbrake-ruby/filters/thread_filter.rb

@@ -38,12 +38,15 @@ def call(notice)
 
       def thread_variables(th)
         th.thread_variables.map.with_object({}) do |var, h|
-          h[var] = th.thread_variable_get(var).inspect
+          h[var] = th.thread_variable_get(var)
         end
       end
 
       def fiber_variables(th)
-        th.keys.map.with_object({}) { |key, h| h[key] = th[key].inspect }
+        th.keys.map.with_object({}) do |key, h|
+          next if (value = th[key]).is_a?(IO)
+          h[key] = value
+        end
       end
 
       def add_thread_info(th, thread_info)

spec/filters/thread_filter_spec.rb

@@ -11,15 +11,15 @@
     subject.call(notice)
     th.thread_variable_set(:bingo, nil)
 
-    expect(notice[:params][:thread][:thread_variables][:bingo]).to eq(':bango')
+    expect(notice[:params][:thread][:thread_variables][:bingo]).to eq(:bango)
   end
 
   it "appends fiber variables" do
     th[:bingo] = :bango
     subject.call(notice)
     th[:bingo] = nil
 
-    expect(notice[:params][:thread][:fiber_variables][:bingo]).to eq(':bango')
+    expect(notice[:params][:thread][:fiber_variables][:bingo]).to eq(:bango)
   end
 
   it "appends name", skip: !Thread.current.respond_to?(:name) do
@@ -49,4 +49,21 @@
     subject.call(notice)
     expect(notice[:params][:thread][:safe_level]).to eq(0)
   end
+
+  context "when an IO-like object is stored" do
+    let(:io_obj) do
+      Class.new(IO) do
+        def initialize; end
+      end.new
+    end
+
+    it "doesn't append the IO object" do
+      expect(io_obj).to be_is_a(IO)
+      th.thread_variable_set(:io, io_obj)
+      subject.call(notice)
+      th.thread_variable_set(:io, nil)
+
+      expect(notice[:params][:thread][:fiber_variables][:io]).to be_nil
+    end
+  end
 end