Use discipline as subject in PDF document properties (resolve #1025)

Author: zepinglee

CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Changed
 
 - `Biblatex` 的斜线不再默认使用等宽字体（[#1018](https://github.com/tuna/thuthesis/discussions/1018)）。
+- 使用“学科”作为 PDF 文档属性的“主题”（[#1025](https://github.com/tuna/thuthesis/issues/1025)）。
 
 ### Fixed
 

thuthesis.dtx

@@ -6527,14 +6527,14 @@
       \hypersetup{
         pdftitle    = \thu@title,
         pdfauthor   = \thu@author,
-        pdfsubject  = \thu@degree@category,
+        pdfsubject  = \thu@discipline,
         pdfkeywords = \thu@keywords,
       }%
     \else
       \hypersetup{
         pdftitle    = \thu@title@en,
         pdfauthor   = \thu@author@en,
-        pdfsubject  = \thu@degree@category@en,
+        pdfsubject  = \thu@discipline@en,
         pdfkeywords = \thu@keywords@en,
       }%
     \fi