refactor src/core/pdf_manager.js: rename pdfModel to pdfDocument

Rename pdfModel to pdfDocument to let the name closer resemble what the
variable actually holds
Add Test for PDFDocumentProxy_getPageIndex
Merge pull request #4481 from bthorben/refactor/pdf_manager.js

refactor src/core/pdf_manager.js: rename pdfModel to pdfDocument