feat: add Action#header method

Closes #42

spec/action_spec.cr

@@ -59,7 +59,7 @@ module Prism::Action
     end
 
     it "sets content type header" do
-      response.content_type.should eq("application/json")
+      response.headers["Content-Type"].should eq "application/json; charset=utf-8"
     end
   end
 
@@ -85,7 +85,7 @@ module Prism::Action
     end
 
     it "sets content type header" do
-      response.content_type.should eq("application/json")
+      response.headers["Content-Type"].should eq "application/json; charset=utf-8"
     end
   end
 
@@ -199,4 +199,20 @@ module Prism::Action
       CallbacksAction.buffer.should eq ["before", "around_before", "call", "around_after", "after"]
     end
   end
+
+  struct HeaderAction
+    include Prism::Action
+
+    def call
+      header("Custom", "42")
+    end
+  end
+
+  describe HeaderAction do
+    response = handle_request(HeaderAction)
+
+    it do
+      response.headers["Custom"].should eq "42"
+    end
+  end
 end

src/prism/action.cr

@@ -121,6 +121,17 @@ module Prism
       context.response.status_code = value
     end
 
+    # Set HTTP header.
+    #
+    # ```
+    # def call
+    #   header("Content-Type", "application/json")
+    # end
+    # ```
+    def header(name, value)
+      context.response.headers[name] = value
+    end
+
     private CONTENT_TYPE_TEXT = "text/html; charset=utf-8"
 
     {% begin %}
@@ -133,7 +144,7 @@ module Prism
       # end
       # ```
       def text(value)
-        context.response.content_type = CONTENT_TYPE_TEXT
+        header("Content-Type", CONTENT_TYPE_TEXT)
         context.response.print(value)
       end
 
@@ -163,7 +174,7 @@ module Prism
       # end
       # ```
       def json(value)
-        context.response.content_type = CONTENT_TYPE_JSON
+        header("Content-Type", CONTENT_TYPE_JSON)
         value.to_json(context.response)
         context.response.close
       end