Add IO flush to builder finish methods

src/http/formdata/builder.cr

@@ -75,6 +75,7 @@ module HTTP::FormData
       @io << "\r\n--" << @boundary << "--"
 
       @state = :FINISHED
+      @io.flush
     end
 
     private def generate_content_disposition(name, metadata)

src/json/builder.cr

@@ -53,6 +53,7 @@ class JSON::Builder
     when DocumentEndState
       # okay
     end
+    @io.flush
   end
 
   def document

src/mime/multipart/builder.cr

@@ -184,6 +184,7 @@ module MIME::Multipart
       end
 
       @state = :FINISHED
+      @io.flush
     end
 
     private def fail(msg)

src/yaml/builder.cr

@@ -72,6 +72,7 @@ class YAML::Builder
   # Ends a YAML stream.
   def end_stream
     emit stream_end
+    @io.flush
   end
 
   # Starts a YAML stream, invokes the block, and ends it.