Fix Grape allowing invalid headers to be set

Fixes ruby-grape#2334

Ensure all header values are strings according to the Rack spec.

* Convert header values to strings using `to_s` in the `header` method in `lib/grape/dsl/headers.rb`.
* Emit a warning if the header value is not a string in the `header` method in `lib/grape/dsl/headers.rb`.
* Add tests in `spec/grape/dsl/headers_spec.rb` to verify that non-string header values are converted to strings and warnings are emitted.

---

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lib/grape/dsl/headers.rb

@@ -10,7 +10,14 @@ module Headers
       # 4. Delete a specifc header key-value pair
       def header(key = nil, val = nil)
         if key
-          val ? header[key.to_s] = val : header.delete(key.to_s)
+          if val
+            unless val.is_a?(String)
+              warn "Header value for '#{key}' is not a string. Converting to string."
+            end
+            header[key.to_s] = val.to_s
+          else
+            header.delete(key.to_s)
+          end
         else
           @header ||= Grape::Util::Header.new
         end

spec/grape/dsl/headers_spec.rb

@@ -56,4 +56,17 @@
       end
     end
   end
+
+  context 'when non-string headers are set' do
+    describe '#header' do
+      it 'converts non-string header values to strings' do
+        subject.header('integer key', 123)
+        expect(subject.header['integer key']).to eq '123'
+      end
+
+      it 'emits a warning if the header value is not a string' do
+        expect { subject.header('integer key', 123) }.to output("Header value for 'integer key' is not a string. Converting to string.\n").to_stderr
+      end
+    end
+  end
 end