Description

XML documents with a <!DOCTYPE declaration cause a warning "Appeared to be invalid xml so was not formatted."

This did not happen in CSharpier 1.1.1

Steps to Reproduce

Example file staff.xml (copied from CSharpier's own test suite):

<?xml version="1.0"?>
<!DOCTYPE staff SYSTEM "staff.dtd"[
    <!ENTITY ent1 "es">
]>
<staff></staff>

$ dotnet csharpier --version
1.2.5
$ dotnet csharpier check staff.xml
Warning ./staff.xml - Appeared to be invalid xml so was not formatted.
Checked 1 files in 79ms.

Expected Behavior

CSharpier should be able to check and format these documents, like in previous versions

Actual Behavior

Warning ./staff.xml - Appeared to be invalid xml so was not formatted.