LibCore: Make IODevice::read_line() return a String

Almost everyone using this API actually wanted String instead of a
ByteBuffer anyway, and there were a bunch of slightly different ways
clients would convert to String.

Let's just cut out all the confusion and make it return String. :^)

Libraries/LibHTTP/HttpJob.cpp

@@ -78,7 +78,7 @@ bool HttpJob::can_read_line() const
     return m_socket->can_read_line();
 }
 
-ByteBuffer HttpJob::read_line(size_t size)
+String HttpJob::read_line(size_t size)
 {
     return m_socket->read_line(size);
 }