Fix: IO::FileDescriptor#seek from current position

Trying to seek from the current position failed because the system
position of a buffered IO is different from the actual position in
the stream.

spec/std/file_spec.cr

@@ -621,6 +621,13 @@ describe "File" do
     file.gets(4).should eq("ello")
   end
 
+  it "seeks from the current position" do
+    file = File.new("#{__DIR__}/data/test_file.txt")
+    file.gets(5)
+    file.seek(-4, IO::Seek::Current)
+    file.tell.should eq(1)
+  end
+
   it "raises if invoking seek with a closed file" do
     file = File.new("#{__DIR__}/data/test_file.txt")
     file.close

src/io/file_descriptor.cr

@@ -131,7 +131,13 @@ class IO::FileDescriptor
     check_open
 
     flush
-    seek_value = LibC.lseek(@fd, offset, whence)
+
+    if whence.current?
+      seek_value = LibC.lseek(@fd, tell + offset, Seek::Set)
+    else
+      seek_value = LibC.lseek(@fd, offset, whence)
+    end
+
     if seek_value == -1
       raise Errno.new "Unable to seek"
     end