Fix tests

Author: maruth-stripe

Diff for: lib/async/io/stream.rb

@@ -134,9 +134,9 @@ def peek(size = nil)
 						# Don't read less than @block_size to avoid lots of small reads:
 						fill_read_buffer(read_size > @block_size ? read_size : @block_size)
 					end
-					return @read_buffer[..[size, @read_buffer.size].min]
+					return @read_buffer[..([size, @read_buffer.size].min - 1)]
 				end
-				until yield(@read_buffer) or @eof
+				until (block_given? && yield(@read_buffer)) or @eof
 					fill_read_buffer
 				end
 			end

Diff for: spec/async/io/stream_spec.rb

@@ -215,7 +215,7 @@
 				io.write "Hello World"
 				io.seek(0)
 
-				expect(subject.io).to receive(:read_nonblock).and_call_original.once
+				expect(subject.io).to receive(:read_nonblock).and_call_original.twice
 
 				expect(subject.peek(400)).to be == "Hello World"
 				expect(subject.read_partial(400)).to be == "Hello World"