BUG: Add whitespace between words for extractText() (#569)

Previously the function .extractText() reads the text in the PDF and returns without any spaces.
In this fix the pdf.py file has been modified to add " " (space) in between two words

Here is an example below:-
Original Sentence : "The quick brown fox jumps over the lazy dog"

Previous Output : "Thequickbrownfoxjumpsoverthelazydog"

After the fix : "The quick brown fox jumps over the lazy dog"

PyPDF2/pdf.py

@@ -2683,6 +2683,7 @@ def extractText(self):
             elif operator == b_("TJ"):
                 for i in operands[0]:
                     if isinstance(i, TextStringObject):
+                        text += " "
                         text += i
                 text += "\n"
         return text