filter/source: add flush() method

src/filter/context/source.rs

@@ -1,3 +1,5 @@
+use std::ptr;
+
 use ffi::*;
 use ::{Error, Frame};
 use super::Context;
@@ -27,4 +29,10 @@ impl<'a> Source<'a> {
 			}
 		}
 	}
+
+	pub fn flush(&mut self) -> Result<(), Error> {
+		unsafe {
+			self.add(&Frame::wrap(ptr::null_mut()))
+		}
+	}
 }