Handle the case of a delimiter starting right after a delimiter look-…

…like

spec/std/io/delimited_spec.cr

@@ -214,6 +214,21 @@ describe "IO::Delimited" do
         io.gets_to_end.should eq("")
       end
 
+      it "handles the case of peek matching first byte, not having enough room, but rest not immediately matching (with a potential match afterwards)" do
+        #                                 not a delimiter, but the second 'f' will actually match the delimiter
+        #                                    ---
+        io = MemoryIOWithFixedPeek.new("abcdeffghijk")
+        #                               -------
+        #                                peek
+        io.peek_size = 7
+        delimited = IO::Delimited.new(io, read_delimiter: "fgh")
+
+        delimited.peek.should eq("abcde".to_slice)
+        delimited.gets_to_end.should eq("abcdef")
+        delimited.gets_to_end.should eq("")
+        io.gets_to_end.should eq("ijk")
+      end
+
       it "handles the case of peek matching first byte, not having enough room, but later matching" do
         #                                  delimiter
         #                                    ---

src/io/delimited.cr

@@ -180,11 +180,12 @@ class IO::Delimited < IO
     # doesn't match the read delimiter's portion, we can move on
     rest = peek[index..]
     unless rest == @read_delimiter[0, rest.size]
-      # If not, we can read from the peek buffer for now
-      slice.copy_from(peek)
-      @io.skip(peek.size)
-      slice += peek.size
-      return peek.size
+      # We can read up to past that byte for now
+      safe_to_read = peek[0, index + 1]
+      slice.copy_from(safe_to_read)
+      @io.skip(safe_to_read.size)
+      slice += safe_to_read.size
+      return safe_to_read.size
     end
 
     # Copy up to index into slice