Change read_to_end to read at least the buffer's capacity.

The buffer's capacity or 16 bytes, whichever is larger, is the minimum
start read size, but it will get twice as large with each read.

src/libstd/io/mod.rs

@@ -345,7 +345,11 @@ fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
     let start_len = buf.len();
     let mut len = start_len;
-    let mut new_write_size = 16;
+    let mut new_write_size = if buf.capacity() > 16 {
+        buf.capacity() - buf.len()
+    } else {
+        16
+    };
     let ret;
     loop {
         if len == buf.len() {