In some scenarios `fetch_text` returns empty string

This issue is related to `schema_salad.fetcher.DefaultFetcher.fetch_text`, and it is a bit difficult to reproduce. I have found it this late afternoon, when I tried next cwltool commands:

```
cwltool --debug --validate https://raw.githubusercontent.com/inab/Wetlab2Variations/eosc-life/cwl-workflows/workflows/workflow.cwl
```
```
cwltool --debug --print-deps https://raw.githubusercontent.com/inab/Wetlab2Variations/eosc-life/cwl-workflows/workflows/workflow.cwl
```

The issue is difficult to reproduce as it has appeared for me only from wireless. But I was unable to reproduce it from my workstation, for instance. When it fails the output is similar to next, but the intermediate stack trace is variable:

```
(.37) jmfernandez@olympo[10]:/tmp/ik> cwltool --debug --validate https://raw.githubusercontent.com/inab/Wetlab2Variations/eosc-life/cwl-workflows/workflows/workflow.cwl
INFO /tmp/ik/.37/bin/cwltool 3.1.20211020155521
ERROR I'm sorry, I couldn't load this CWL file.
The error was:
Traceback (most recent call last):
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/main.py", line 1067, in main
tool = make_tool(uri, loadingContext)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/load_tool.py", line 470, in make_tool
tool = loadingContext.construct_tool_object(processobj, loadingContext)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/workflow.py", line 51, in default_make_tool
return Workflow(toolpath_object, loadingContext)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/workflow.py", line 106, in __init__
step, index, loadingContext, loadingContext.prov_obj
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/workflow.py", line 150, in make_workflow_step
return WorkflowStep(toolpath_object, pos, loadingContext, parentworkflowProv)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/workflow.py", line 234, in __init__
self.embedded_tool = load_tool(toolpath_object["run"], loadingContext)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/load_tool.py", line 486, in load_tool
loadingContext, workflowobj, uri = fetch_document(argsworkflow, loadingContext)
File "/tmp/ik/.37/lib/python3.7/site-packages/cwltool/load_tool.py", line 133, in fetch_document
loadingContext.loader.fetch(fileuri, content_types=CWL_CONTENT_TYPES),
File "schema_salad/ref_resolver.py", line 985, in fetch
StopIteration
```

Digging in cwltool and later in schema_salad I found that, sometimes next block returns what it is expected, sometimes it returns an empty string:

https://github.com/common-workflow-language/schema_salad/blob/5d45ae6cdc4b9896d6e305b4372cb8aef3e42238/schema_salad/fetcher.py#L59-L75

It seems some weird behavior in `requests` library, where it does not materialize the full response of the request when the property is read (but I'm guessing, as I'm not a frequent user of such library). I guess that because I did a quick'n'dirty test, substituting `requests` library calls by `urllib.request` ones (see code below), and it started working:

```python
if scheme in ["http", "https"] and self.session is not None:
try:
headers = {}
if content_types:
headers["Accept"] = ", ".join(content_types) + ", */*;q=0.8"
req = urllib.request.Request(url, headers=headers)
#resp = self.session.get(url, headers=headers)
#resp.raise_for_status()
resp = urllib.request.urlopen(req)
except Exception as e:
raise ValidationException(f"Error fetching {url}: {e}") from e
if content_types and "content-type" in resp.headers:
content_type = resp.headers["content-type"].split(";")[:1][0]
if content_type not in content_types:
_logger.warning(
f"While fetching {url}, got content-type of "
f"'{content_type}'. Expected one of {content_types}."
)
raw_content = resp.read()
resp.close()

encoding = resp.headers.get_content_charset()
return raw_content.decode('utf-8' if encoding is None else encoding)
# return resp.text
```

Hope this helps to anyone else having this issue!